Japanese Patent Application Publication No. 2008-104274 (especially, FIG. 1 and paragraph [0026]) discloses an LED (light emitting diode) lighting device for lighting LEDs by power-converting a DC voltage, which has been obtained by rectifying and smoothing an AC voltage, by a DC-DC converter. The lighting device includes an inrush current preventing circuit in which a parallel circuit of a current limiting resistor and a thyristor is positioned between a rectifying circuit and a smoothing capacitor. An inrush current to the smoothing capacitor is restrained at an initial stage of power input by the current limiting resistor and the thyristor is turned on in case of normal lighting to thus restrain a loss of power due to the current limiting resistor. In addition, this technique discloses a configuration in which a gate driving current of the thyristor of the inrush current preventing circuit is supplied by rectifying and smoothing a flyback current which is obtained from an intermediate tab of an inductive element of the DC-DC converter.
In the foregoing patent document, the DC-DC converter has a configuration in which a serial circuit of a switching element and a primary coil of a transformer is connected in parallel to the smoothing capacitor. Thus, one end of the primary coil of the transformer as an inductive element can be connected to a cathode of the thyristor of the inrush current preventing circuit and a voltage of the capacitor charged through a diode from the intermediate tab of the primary coil can be supplied between the gate and the cathode of the thyristor. Same configuration may also be used also in a boosting chopper circuit.
Meanwhile, when the DC-DC converter is a step-down chopper circuit, the inductor as an inductive element is normally connected to the smoothing capacitor serving as an input DC power source through a load circuit or a switching element, rather than being directly connected thereto. This is because there is a necessity to configure a closed circuit by the inductive element, the load circuit, and a flyback diode in the step-down chopper circuit. Therefore, if the inductive element is directly connected to the input DC power source, the load circuit is separated from the potential of the input DC power source.
In this case, the potential of the load circuit is changed in an RF manner; and thus, for example, when it is used in a power source separation type LED lighting device, e.g., as shown in FIG. 5, it causes RF radiation noise. Thus, in such configuration, in particular, one end of the load circuit is required to be directly connected to the input DC power source to stabilize the potential, and as a result, an inductor L1 as an inductive element is connected to the input DC power source through the load circuit or the switching element as illustrated in FIGS. 1 to 3. For this reason, when the DC-DC converter is a step-down chopper circuit, the configuration of above-described cannot be applied as it is.